Protective-colloid-stabilized polymers are used especially in the form of their aqueous dispersions or water-redispersible polymer powders in a wide variety of applications, for example as coating compositions or adhesives, for a very wide variety of substrates. The protective colloids used generally comprise polyvinyl alcohols. The use of polyvinyl alcohol is desirable because, in contrast to systems stabilized by low-molecular-weight compounds (emulsifiers), this material itself contributes to strength (e.g. tensile bond strength values in tile adhesives). Preferred monomers for producing redispersible powders have hitherto been vinyl esters and ethylene, since it is not easy to achieve stabilization of acrylate copolymers or styrene acrylate copolymers using polyvinyl alcohol.
The traditional production of said dispersions uses a batch emulsion polymerization process. This procedure features high flexibility and is therefore preferred in plants with a wide variety of products.
A disadvantage is that the large amounts of energy generated during the polymerization reaction cannot be adequately dissipated by cooling assemblies internal to the reactor. Cooling assemblies internal to the reactor are generally a jacket within which coolant flows around the reactor, or cooling coils attached on the inner wall of the reactor. The result of the limited amount of heat dissipation that said equipment can provide is sometimes very long process times, which adversely affect the economics of the process. A known method of improving heat dissipation and accelerating the process is to conduct the polymerization in such a way that the polymerizing medium is introduced via a circuit to an external cooler, from where it is returned to the reactor.
EP 0 834 518 A1 describes a process for producing polymer dispersions by means of emulsion polymerization and dissipating the heat of polymerization by way of an external cooler, characterized by use of a heat exchanger with a substantially laminar flow profile, and of a low-shear pump. The presence of laminar flow is particularly emphasized and serves to minimize the effect of shear forces. WO 03/006510 A2 describes a similar process, except that there is specific recommendation of tubular diaphragm pumps that provide low-shear conveying action. WO 02/059158 A1 describes a process where a portion of the monomer is introduced directly into the external circuit. This is intended to help reduce formation of coagulate and of deposit on the wall. EP 0 608 567 A1 relates to a process for polymerizing vinyl chloride in a reactor with an external cooling circuit which has been equipped with heat exchanger and with a pump, where the pump used comprises a Hidrostal pump with screw centrifugal impeller. EP 0 526 741 A2 discloses a reactor system which has an external cooling circuit and which is characterized by a specific pump, with an impeller in the form of a spiral screw blade. DE 199 40 399 recommends using an impeller pump to convey the polymer dispersions in the cooling circuit.
EP 1174445 A1 describes continuous emulsion polymerization in at least two pressure reactors arranged in series and subsequently in at least one unpressurized reactor, this polymerization being initiated by means of a redox system, and at least some of the reduction component being introduced in the first pressure reactor. EP 1067147 A2 describes a continuous process for emulsion polymerization in which a low-molecular-weight polyvinyl alcohol is used as protective colloid. EP 1384502 A1 discloses a static mixer-heat exchanger which encompasses a housing and, arranged therein, tubes which are supplied with heat-transfer medium, where the tubes have connecting fillets which bring about the static mixing effect when a substrate is passed through the housing. US 2001/0012235 A1 describes a static mixer made of an enclosing housing and of a mixer insert, where the mixer insert is composed of a large number of interlocking grids. EP 0755945 A1 describes an apparatus which encompasses a tubular reactor and a circuit for the partial return of polymerization product. There are two mixer-heat exchangers arranged within the circuit: one on the outgoing side, which heats the polymerization product, and one on the ingoing side, which in turn cools the polymerization product. WO 00/71319 A1 describes an apparatus in which product is removed from a reactor and introduced into an extruder, where, in the section between reactor and extruder, the product is cooled in heat exchangers arranged in series.
A disadvantage is that if the dimensions of conventional heat exchangers (plate heat exchangers, tube-bundle heat exchangers, spiral heat exchangers) are kept within reasonable limits they have only limited heat-dissipation capacity. One of the reasons for this is that the dispersions to be cooled give only laminar flow. To achieve tolerable cooling rates it therefore necessary to pump material through the external circuit with very high flow rates. The high-performance pumps necessary for this can cause enormous damage to the product, by introducing energy into the medium.
It was an object of the present invention to provide a more efficient process for heat dissipation during emulsion polymerization reactions, without any adverse effect on the properties of the product.